The invention is related to an exhaust purification catalyst for a compression ignition engine.
An exhaust purification catalyst is known for purifying HC, CO and SOF included in an exhaust gas discharged from a compression ignition:)engine (Japanese Unexamined Patent Publication No. 5-57191). The exhaust purification catalyst comprises a carrier substrate, and, an activated alumina layer carries catalytic metal, for purifying HC, CO and SOF, on a surface of the carrier substrate. Further, SO2 is included in the exhaust gas, and toxic SO3 is produced when SO2 reaches the catalytic metal and is oxidized. Thus, according to the above exhaust purification catalyst, in order to prevent SO2 from reaching the catalytic metal, a trap layer for trapping SO2 is provided upstream of the activated alumina layer. The trap layer is made of an alumina layer including, for example, Mn oxide therein.
In the above exhaust purification catalyst, the amount of SO2 trapped by the trap layer is limited. Further, there is a possibility that the trap layer may become unable to trap SO2 due to the deterioration thereof. Therefore, there is a problem that it easily becomes impossible to prevent SO2 from reaching the catalytic metal. Thus, the object of the invention is to prevent SO2 from reaching the metal-carrying layer.
In the first invention, an exhaust purification catalyst for a compression ignition engine comprises a carrier substrate and a metal-carrying layer which carries catalytic metal formed on the surface of the carrier substrate and is characterized in that a diffusion restriction layer having a thickness less than 170 xcexcm is provided on the surface of the metal-carrying layer opposite to the carrier substrates. The diffusion restriction layer provided on the surface of the metal-carrying layer prevents substances included in the exhaust gas from diffusing into the metal-carrying layer.
In the second invention, an exhaust purification catalyst for a-compression ignition engine comprises a carrier substrate and a porous metal-carrying layer which carries catalytic metal formed on the surface of the carrier substrate and is characterized:in that a porous diffusion restriction layer which includes pores having sizes smaller than those of the metal-carrying layer is provided on the surface of the metal-carrying layer opposite to the carrier substrate. The diffusion restriction layer which has pores having sizes smaller than those of the metal-carrying layer prevents substances included in the exhaust gas from diffusing into the metal-carrying layer.
In the third invention according to the first or second invention, catalytic metal which has an oxidation power lower than that of the catalytic metal carried on the metal-carrying layer is carried on the diffusion restriction layer.
Therefore, the substances included in the exhaust gas are oxidized in the diffusion restriction layer.
In the fourth invention according to the first or second invention, a catalytic metal which traps SO2 is carried on the diffusion restriction layer. SO2 included in the exhaust gas is trapped by the diffusion restriction layer so that SO2 is further restricted from diffusing into the metal-carrying layer.
In the fifth invention according to the first or second invention, a diffusion restriction layer is provided between the carrier substrate and the metal-carrying layer. Therefore, the chance that substances included in the exhaust gas stay in the vicinity of the metal-carrying layer is increased.
In the sixth invention according to the first or second invention, an adsorbing layer is provided for adsorbing hydrocarbon between the carrier substrate and the metal-carrying layer. Therefore, the chance that HC included in the exhaust gas stays in the vicinity of the metal-carrying layer is increased.
In the seventh invention according to the first or second invention, a portion having a predetermined thickness of the diffusion restriction layer which extends from the surface of the diffusion restriction layer opposite to the metal-carrying layer into the diffusion restriction layer comprises a thermal deterioration restriction layer which has thermal resistance.
In the eighth invention according to the seventh invention, the thermal deterioration restriction layer is made of alumina, and the remaining diffusion restriction layer is made of titania.